TOKYO - A strong earthquake rocked eastern Japan late today, shaking buildings in Tokyo and nearby areas, but there was no immediate report of injuries or damage.

The earthquake, with a preliminary magnitude of 6.5, was centered 24 miles below the sea off the coast of Ibaraki prefecture, the U.S. Geological Survey said. There was no danger of tsunami, it added.

Runways at Tokyo's Narita airport closed temporarily but later reopened, Kyodo News agency said. Bullet train service north of Tokyo also was suspended but quickly resumed.

A nuclear power plant in Tokaimura near the quake zone had shut down automatically, according to national broadcaster NHK.

Japan is one of the world's most earthquake-prone countries because it sits atop four tectonic plates. Since the late 1970s, the government has taken measures to strengthen its monitoring of seismic activity, and to coordinate steps with local governments on how to cope with earthquakes.